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Haze

With the Clean Air Act of 1977, Congress established a national goal of remedying existing, and preventing future, human-caused visibility impairment in the larger Wilderness Areas, National Parks, and National Wildlife Refuges that existed at the time. Air pollution likely impairs visibility to some degree on all federal lands. The visual range within the eastern U.S. is often just 15 to 30 miles, estimated at one-third of what it would be without human caused air pollution. In the West, the visual range averages between 60 and 90 miles, or about one-half of the visual range under natural conditions. Extensive data and visibility information can be found at http://views.cira.colostate.edu/web/.

Haze is caused by fine particles in the air that scatter and absorb light. When the amount of fine particles increases, more light is absorbed and scattered, resulting in a shorter visual range, less clarity and altered color.

Five types of fine particles contribute to haze: sulfates, nitrates, organic carbon, elemental carbon, and crustal (soil) material. The relative impact of each type of particle varies across the U.S. and from season to season.

Plume Blight

Plume blight occurs when a point source such as a smoke stack emits particulate matter or nitrogen dioxide into a stable atmosphere. These pollutants can form a thin, dark, coherent plume obscuring the view. This picture captures a classic example of plume blight. Blight occurs before the plume has been dispersed so widely that it is indistinct from the background. Both contrast and coloration may vary depending upon the plume constituents, the viewing background, the viewer angle, and the angle of the sun.

Contribution of Various Particulates to Haze

Sulfate particles form in the air from sulfur dioxide gas. Most of this gas is released from industrial sources such as coal-burning power plants, smelters, and oil refineries. Sulfates are the largest contributor to haze in the eastern U.S. In humid environments, sulfate particles grow rapidly to a size that is very efficient at scattering light.

Nitrate particles form in the air from nitrogen oxide gas. This gas is released from virtually all combustion activities, especially those in power plants, oil and gas production, and other industrial sources as well as in cars, trucks, and motors like those in lawn mowers and boats. Like sulfates, nitrates scatter more light in humid environments.

Organic carbon particles are emitted into the air and also form there as a reaction of various gaseous hydrocarbons. Major sources of organic carbon particles include vehicle exhaust, vehicle refueling and solvent evaporation. Hydrocarbon emissions from forests and wildland fire smoke are additional sources. Fire emissions also include primary organic particles in the form of uncombusted material.

Elemental carbon particles like soot are smaller than most other particles and absorb rather than scatter light. The brown clouds often seen in winter over urban areas and some mountain valley towns can be largely attributed to elemental carbon. These particles are emitted directly into the air from virtually all combustion activities. They are especially prevalent in diesel exhaust and smoke from the burning of wood and wastes.

Crustal material, like dust, enters the air from dirt roads, fields, and other open spaces as a result of wind, traffic, and other surface disturbing activities. For more information about impacts to visibility, visit http://www.fsvisimages.com/viscause.aspx.

To reduce haze, emissions of haze-forming pollutants across broad areas of the country must be reduced. Cars, trucks, and industries are much cleaner than they were in the past, and programs are in place to maintain this progress into the future. However, these programs are probably insufficient to restore visibility to its natural conditions in many protected areas.

The Forest Service is working closely with each of the 50 states as they develop plans to reduce regional haze impacting large wilderness areas and national parks which were in existence prior to the Clean Air Act. For more information, click here.

Visibility, Safety, and Nuisance

Introduction of particulate matter and certain gases into the atmosphere interferes with the ability of an observer to see landscape features. Fine particulates (PM-2.5) that are prevalent in smoke are more efficient at scattering and absorbing light and impairing visibility than coarse particulates (PM-10). Smoke can cause short-term visibility impacts by obscuring the form, contrast detail, and color of near and distant features. It poses a safety hazard when it obscures visibility on land, water, or aerial transportation routes. Smoke can also cause public nuisance and complaints about loss of visibility, odor, and soiling from ash fall-out at relatively low pollutant concentrations.

For more information on fire emissions, modeling, and monitoring, visit the regional planning organization website in your area, Smoke management: http://www.nifc.gov/smoke/ or http://www.nwcg.gov/branches/ppm/smoc/index.htm.